The kinetic mechanism of SS isozyme of horse liver alcohol dehydrogenase is shown by initial velocity and product inhibition studies to be asymmetrical, being random for ethanol oxidation and compulsory ordered for acetaldehyde reduction. Enzyme isomerization seems to account for the asymmetry in the mechanism. In its interaction with NADH, the SS isozyme resembles classical alcohol dehydrogenase; consequently, the maximal velocity in the direction from ethanol to acetaldehyde appears to be determined by the rate of NADH dissociation. In the direction from acetaldehyde to ethanol, the enzyme isomerization step appears to limit the maximal velocity.